It is becoming increasingly important to reduce the size of radio equipment to enhance its portability. For example, the smallest available cellular telephone handset today can conveniently fit into a shirt pocket or small purse. In fact, so much emphasis has been placed on obtaining small size for radio equipment that corresponding antenna gains are extremely poor. For example, antenna gains of the smallest handheld phones are only −3 dBi or even lower. Consequently, the receivers in such phones generally do not have the ability to mitigate interference or reduce fading.
Some prior art systems provide multiple element beam formers for these purposes. These antenna systems are characterized by having at least two radiating elements and at least two receivers that use complex magnitude and phase weighting filters. These functions can be implemented either by discrete analog components or by digital signal processors. The problem with this type of antenna system is that performance is heavily influenced by the spatial separation between the antenna elements. If the antennas are too close together or if they are arranged in a sub-optimum geometry with respect to one another, then the performance of the beam forming operation is severely limited. This is indeed the case in many compact wireless electronic devices, such as cellular handsets, wireless access points, and the like, where it is very difficult to obtain sufficient spacing or proper geometry between antenna elements to achieve improvement.
Indoor multipaths, mostly outside the main beam, interfere with the main beam signal and create fading. The indoor multi paths also create standing wave nulls that prevent reception if the directive antenna is situated at these nulls. For a traditional array, if one element of the array is at the null, the received signal is still significantly reduced. Reciprocity makes this effect hold true for the transmit direction, too.